An optical assay device is a device used to detect an analyte such as an antigen. These devices may carry an optically active test member to which a sample is applied for determining the presence or amount of an analyte of interest.
It is desirable in an assay device for the optically active test member to be extremely sensitive to the existence of an analyte, and for the assay performance time, i.e., incubation time, to be as short as possible. This is accomplished in flow-through optical assay devices by maximizing the sample volume which is brought in contact with an analyte specific receptive material or the test member and controlling the flow characteristics of the sample through the optical member.
Although the sample will flow through the optical member without external assistance, the flow characteristics of the sample across the optical member and through channels within the optical member or around the optical member can be modified by the use of absorbent materials. Absorbent material allows for wicking which acts to draw fluid from the surface that the adsorbent material is in contact with which can cause fluid to be drawn across the layers of the optical member and through the channels within the optical member or around the optical member. The absorbent material also provides drying of the optical member when contacted with the optical stack. This drying helps to distinguish the signal produced by the optical member.
U.S. Pat. No. 5,418,136 (Miller et al.) describes a blotting device and blotting method which uses an optically reactive surface as the receptor for samples and reagents related to the particular assay being performed. The device contains an optically reactive layer supported on a pedestal of the device in order to allow placement of various solutions, e.g. sample, washing reagents, substrate, directly onto the reactive layer's top surface. The solutions are removed by blotting the reactive surface with an absorbent material by physically pressing the absorbent material onto the reactive surface.
These optical assay devices require that the user apply a discrete volume of sample (approximately 25-30 .mu.L) on the surface and that the incubation times be controlled by user intervention. Sample incubates on the surface in a static mode as the surface is solid and impermeable. The drying process also requires user interaction to bring the adsorbent material into contact with the solid optical test surface from above the test surface. While the solid surface optical assays are extremely sensitive, an improvement in sensitivity can be gained by using all of the available sample (dependent on sample processing but generally greater than 200 .mu.L). In many testing sites, the requirement for user intervention in timing and drying the optical test device is inconvenient and not cost effective.
The prior art also includes assay devices that allow for sample flow through the surface of a porous material or across a tortuous path material. Detection is based on the generation of a colorimetric signal through the use of a chromophore or a light scattering particle and signal generation is external to and independent of the surface characteristics of the porous support. In these assays, sample flows through the device with a very limited contact time with the capture element of the device. Thus, sensitivity of the assay is limited by the capture efficiency of the system. Many of these devices suffer from highly variable flow rates as minor changes in the sample composition occur.
The devices of the current invention allow the sample incubation to occur over a period of time to improve capture efficiency but also minimize the user intervention required to complete the assay. The devices also provide an increase in assay performance by allowing all available sample to flow across the optical member and through channels within the optical member. Because the contact time of sample with the test surface is controlled, the devices are less sensitive to variable flow rates than other prior art devices. Also in the devices of this invention, the signal generation is inherent in the composition and construction of the flow through support. Drying of the optical surface from below instead of from above decreases the risk of damaging the optical surface prior to the detection step.